In basic and applied research, stainless steel manifolds and implements are used increasingly for high vacuum, and ultra-high vacuum apparatuses. Depending on the cleanliness and vacuum to be achieved bake-out is a requisite, and a well known routine procedure in this art. The temperature necessary varies from 200.degree. to 400.degree. centigrade depending on the most heat-sensitive material within the vessel. Above approximately 300.degree. the stainless steel wall material, usually type S-304 begins to oxidize when exposed to atmospheric air, as evidenced by a gradually deepening yellow surface tint of the manifold.
This superficial, and external, oxidation of stainless steel does not impair the proper functioning of the manifold, and attached implements. However, the internal surface that faces and surrounds the assay to be investigated may exert a marked impact on that assay. Stainless steel is basically an alloy of iron, chromium and nickel that also contains carbon in the form of graphite, up to two percent of manganese, one percent silicon, and last but not least contaminants such as phosphorus and sulfur. While all the basic constituents of stainless steel exhibit, at any temperature, vapor pressures of roughly the same order of magnitude, respectively. For reasons that will become evident, such constituents will be disregarded in this disclosure. In contrast, the vapor pressure of manganese is three orders of magnitude higher than that of iron, chromium or nickel while phosphorous and sulfur are, about 10-15 orders of magnitude higher at the same temperature.
The above facts show that the use of high vacuum and especially ultra-high vacuum manifolds made of commercially available stainless steel represents a distinct hazard to ultra-clean processing of solid state samples or assays. One example of such a sample is a photocathode which is made from very pure semiconductor materials. It is well known that assays of the materials sold for research programs on the above devices run to purities of 99.999 percent or better. The addition of a few parts per million from some of the contaminates mentioned above cause measureable modifications in the electrical properties of the product devices.